Semiconductor devices, such as flash memory devices as well as other types of memory devices, continue to increase in device density and speed while decreasing in size and power consumption. However, semiconductor devices, such as flash memory devices, having increased density and speed and decreased size and power consumption also require highly conductive and reliable interconnect lines. Consequently, copper (“Cu”) metallization has become more desirable as a replacement for (“Al”) based metallizations, since copper has a lower resistance, i.e. it is more conductive, as compared with aluminum. The lower resistance of copper enables signals in the semiconductor device to move faster by decreasing the RC time delay in the device's interconnect lines. Furthermore, since copper has a higher electromigration resistance compare to aluminum, copper interconnect lines can reliably handle higher current densities with thinner lines. However, copper is difficult to use in semiconductor manufacturing, since it, i.e. copper, diffuses very quickly in silicon and can damage active devices if it reaches the active area of the semiconductor die. As a result, copper interconnect lines must be surrounded by a diffusion barrier layer to block undesirable copper diffusion. Also, the particular characteristics of copper, such as copper's ability to readily oxidize, makes bonding to copper much more difficult than bonding to aluminum or gold. As a result, semiconductor manufacturers are challenged to provide effective copper bond pad structures for semiconductor devices, such as flash memory devices, which utilize copper metallization.
In a conventional bond pad structure utilized in copper metallization, a terminal metal structure comprising a layer of aluminum is situated over a diffusion barrier layer, which typically comprises tantalum (“Ta”), is formed over a copper metal pad. The layer of aluminum provides a reliable surface that readily accepts wire bonds, while the diffusion barrier layer effectively prevents copper from migrating to the layer of aluminum. However, during the stress of wire bonding, the diffusion barrier layer may develop cracks, which can allow copper to migrate to the aluminum layer and cause a bonding failure.
Thus, there is a need in the art for a more reliable bond pad structure for semiconductor devices, such as flash memory devices, utilizing copper metallization.